The iPad is not an iPhone. People have to make a specific case to buy the iPad, where as 99.9% of us need an iPhone. For that reason the sales and uptake of the device won't be nearly as fast.. and it will take a bit of time to develop those "killer apps" that really drive sales of the device.
Some people out there though seem to be a bit impatient and expect the App sales numbers to be on par with the iPhone. It's a stretch, that device has been out for years and has a huge consumer base. The iPad however has only been available for a few days.
This article by the SFGate talks a bit about some disappoint app sales numbers on release and goes into some detail about it:
The Reason No One Is Bragging About iPad App Sales: The Numbers Are Too Small (AAPL)